FIG. 1 shows schematically networks between a central office and subscribers such as a point-to point optical telecommunications network and a point to multipoint optical telecommunications network having a splitter and branch fibers. A passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32-128. A PON consists of an Optical Line Terminal (OLT and a number of Optical Network Units (ONUs).
Network operators have a need for test equipment (like OTDR equipment) that is capable of monitoring connected and unconnected lines from a central office during installation and during maintenance. Preferably access to the far ends of the network for such monitoring should not be required, e.g. without truck rolls. This requirement effectively excludes any method which requires an optical source at one end of the fiber and a detector to be placed at the other. Also, preferably, the measurements should not disturb the data traffic. For this reason, there is a requirement that additional equipment, e.g. standard or calibrated reflectors or detectors, are not attached and detached from the optical fiber, either at the head end or remote ends of the fiber network.
One problem is that the interpretation of the OTDR measurements is quite complicated or sometimes even impossible on point to multipoint networks for the following reasons. The measured loss in a fiber network depends on the number of splitter ports, the number of drop cables spliced to the splitter and even the location where the loss occurs in the drop cable. This makes it extremely complicated to monitor the change in attenuation of each line, certainly when the network changes over time. For example, the same bend in the fiber at different locations will give different measured loss values when measured with an OTDR. Also as the number of branch fibers increases, the signal from each fiber gets weaker. Conventional systems use the backscattered optical power information of the fibers, i.e. the noisy baseline of OTDR trace. Beyond the splitter the signal becomes noisy due to the lack of dynamic range. No accurate information can be obtained from this noisy baseline. For splitter levels above 1×8 often no information at all can be obtained from the baseline. For example, with a 1 to 32 splitter, a 3 dB loss in a 1×32 PON network can give measured values ranging from 0.02 dB to 3 dB depending the location of the loss and the length of the other drop lines. This can means that it is not possible to set alarms reliably.
Conventional OTDR has the following problems:                Substitution of traces from all ports        For 1:32 splitter ratio's the received power from a single port is close to the noise level of the test equipment. For example, OTDR has only 20-25 dB dynamic range without averaging and short pulses.        Measured splice loss for a splice located after the splitter depends on splitter port count and location in the drop cable.        